ASoC: ad193x: Split SPI and I2C code into different modules

There are a few known (minor) problems with having the support code for both I2C
and SPI in the same module:
    * We need to be extra careful to make sure to not build the driver into the
      kernel if one of the subsystems is build as a module (Currently only I2C
      can be build as a module).
    * The module init path error handling is rather ugly. E.g. what should be
      done if either the SPI or the I2C driver fails to register? Most drivers
      that implement SPI and I2C in the same module currently fallback to
      undefined behavior in that case. Splitting the the driver into two
      modules, one for each bus, allows the registration of the other bus driver
      to continue without problems if one of them fails.

This patch splits the AD193X driver into 3 modules. One core module that
implements the device logic, but is independent of the bus method used. And one
module for SPI and I2C each that registers the drivers and sets up the regmap
struct for the bus.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>